Device and method for calibrating and improving the accuracy of barometric altimeters with gps-derived altitudes

ABSTRACT

In a combined GPS/altimeter device, the calibration and hence the accuracy of barometric altimeter measurements are enhanced with the aid of derived altitudes from a GPS. The device determines the need for calibration and perform the subsequent computations necessary to facilitate the calibration. Furthermore, the device determines a correction quantity that should be applied to barometric altitude readings, thereby allowing the device to be calibrated while in motion. Both of these features ultimately result in a more accurate determination of altitude.

BACKGROUND OF THE INVENTION

This application is a continuation, and claims priority benefit, ofapplication Ser. No. 10/826,754, filed Apr. 16, 2004, which is acontinuation of application Ser. No. 10/299,932, filed Nov. 19, 2002,now U.S. Pat. No. 6,768,449, which is a divisional of application Ser.No. 09/833,318, filed Apr. 12, 2001, now U.S. Pat. No. 6,522,298. Theabove applications are incorporated herein by specific reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to enhancements for incorporation intoan electronic navigation device. More particularly, the presentinvention is directed to an enhancement of the calibration and hence theaccuracy of barometric altimeter measurements with the aid of derivedaltitudes from a global positioning system.

2. Description of the Related Art

In general, altitude measurements are made using two methods ofmeasurement. One method utilizes a barometric altimeter. Barometricaltimeters are devices that sense local atmospheric pressure and use astandard model of the atmosphere to convert this pressure measurementinto altitude. Altitude measurements are referenced to height above meansea level (MSL).

It is well known that local atmospheric pressure at a given altitudevaries widely due to the effects of weather, solar heating, and otherfactors. Thus, in order to provide an accurate altitude, barometricaltimeters must be calibrated to correct for these variations. TheGlobal Positioning System (GPS) is a worldwide navigation system thatcan determine a user's position in horizontal and vertical dimensions.However, GPS vertical measurements are currently all referenced to theWGS-84 ellipsoid, a purely mathematical construct that approximates theshape of the earth. The GPS receiver must use a model that relates theheight above the ellipsoid to the height above mean sea level.

Further, it is well known that the vertical measurement of a GPS systemis inherently less accurate than the horizontal measurements. This isdue to the fact that GPS satellites are constrained to be above thehorizon for signal reception to occur. This geometry is less thatoptimal for measuring the vertical component of a user's location simplybecause there can not be satellites visible below the user (an optimalconfiguration would have satellites above and below the user). All GPSreceivers are able to take into account satellite geometry (Dilution ofPrecision) and estimates of other satellite-related errors (URA) andprovide a statistical estimate of the errors in the horizontal andvertical measurements.

In practice, a barometric altimeter typically provides a more stablemeasurement of altitude than GPS over short time periods. However, overlong time periods, pressure variations can be of such magnitude that thebarometric altimeter measurement of altitude is less accurate than theGPS measurement. The pressure-indicated altitude of an uncalibratedbarometric altimeter is typically in error by many tens of meters due tonormal atmospheric pressure fluctuations, weather fronts and othersources. However, this error is of a bias like nature—it is slowlyvarying with time—resulting in less accurate barometric altimeterreadings over long time periods. Accordingly, while an altitudedetermination derived from barometric pressure may be meaningfullyaccurate in a short time frame, over time, the accuracy of such adetermination becomes undesirable. Conversely, because GPS derivedaltitude suffers from different complementary errors, over a short timeperiod (typically minutes time frame), GPS altitude measurements aresubject to much larger variations than barometric altimetermeasurements.

In an attempt to overcome the foregoing, one proposal combines a GPSunit and a barometric pressure sensor in the same housing. However, inthat proposal the pressure sensor is used to augment GPS derivedaltitude information.

In particular, McBurney et al., in U.S. Pat. No. 6,055,477, disclose amethod of combination or integration of measurements made using twosystems to provide better availability or accuracy in altitudemeasurements by estimating a barometric bias using the difference inaltitude obtained from the two sources. The McBurney et al. methodhowever, fails to recognize that utilizing the difference between a GPSderived altitude and a barometric altimeter altitude, as a term incalibrating the barometer altimeter, will common mode out any dynamicchanges due to movement of the barometric altimeter and the GPS intandem. As a result, the McBurney et al. approach undesirably requiresthat the user not change altitude during calibration periods.

Additionally, in the stated prior approach, an altimeter may only becalibrated using GPS derived altitude information when the user isstationary, often referred to as a “Calibration mode”. The presentinvention makes no distinction between “calibration mode” and“navigation mode”, indeed the barometric error is constantly beingestimated and used to calibrate the system. Furthermore, the presentinvention provides a method to statistically determine the need forcalibration which results in both the calibration and error estimationnumbers being calculated and utilized without any user intervention,i.e. the user need not place the device in ‘calibration mode’ to obtainthe required bias parameter for calibration computations.

There exists a need for a method to take advantage of the long termstability of the GPS altitude measurement and the short term stabilityof the barometric altimeter measurement to produce an altitudemeasurement that is stable and accurate over long and short timeperiods. Additionally, the need exists for a method that uses bothGPS-derived altitude and barometric altimeter altitude to produce analtitude measurement that is more stable and accurate than eithermeasurement taken alone. The need also exists for an improved method tocalibrate a barometric altimeter and to compute a barometric altitudecorrection quantity. Particularly, the need exists for a method to haveGPS and altimeter outputs to be self calibrating while the user is onthe move. The present invention fills the foregoing identified needs,and other needs, while overcoming the drawbacks of the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvednavigation unit.

It is another an object of this invention to compute a barometricaltitude correction quantity such that the use of this quantity duringcalibration results in measurements that are more stable and accuratethan GPS or barometric measurements taken independently.

It is another object of the present invention to improve on constraintsthat are required by prior art methods of various types where there is auser distinction between calibration and navigational modes.

It is a further object of the invention to provide unique options forstatistically determining the need for calibration of an altimeter basedon discrepancy between GPS altitude measurements and other altitudemeasurements.

It is another object of the present invention to provide continuouscalibration of an altimeter while the unit is on the move.

These and other objects are achieved by a portable unit having aninternal processor. Connected to the processor are, at a minimum, aninput (such as a keypad), a display, a memory, a barometric pressuresensor, and a GPS receiver, which also connects to an antenna, such as aGPS patch antenna. These components, along with a power supply (such asbatteries) are housed within a housing. As will be understood andappreciated, the input and display are accessible at an exterior of thehousing, in a conventional manner.

A navigation device incorporating the present invention serves as a GPSunit, in that GPS signals from a plurality of satellites may be receivedby the GPS receiver, such that the processor calculates positioninformation based upon the received signals. The conventional use andoperation of GPS units is well known, and need not be further described.

Additionally, the present invention addresses an altimeter. Inparticular, the barometric pressure sensor measures barometric pressureand provides the sensed barometric pressure information to theprocessor. The processor, utilizing stored software, then converts themeasured pressure into an altitude, which may be displayed or otherwisecommunicated to the user. The conversion of barometric pressure toaltitude may be accomplished in any desired and conventional manner. Forexample, a lookup table may be provided in the memory, where the tablecontains altitude information corresponding to known barometricpressures. Thus, an altitude corresponding with a sensed barometricpressure may be retrieved from memory and displayed on the display.Alternatively and preferably, altitude (or elevation) may be calculatedusing a known equation.

In particular, the present invention provides a unique navigation deviceand method for a navigation device that combines data from a pluralityof sensors and position information obtained from a GPS, to provide theuser with an accurate representation of altitude information.Additionally, as stated, the simultaneous access to GPS information andaltimeter information, as well as calculating the difference betweenthat information to obtain an indication of bias, allows for featuressuch as automatic calibration and calibration while the user is on themove.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention noted above are explained inmore detail with reference to the drawings, in which like referencenumerals denote like elements, and in which:

FIG. 1 is a front view of an illustrative embodiment of a navigation ofthe present invention;

FIG. 2 is an illustrative block diagram of a navigation device thatincorporates the present invention;

FIG. 3 is a graphical representation of a user's elevation trajectoryand GPS based and barometric altimeter based elevation readings;

FIG. 4 is a block diagram illustrating barometric altimeter calibrationprocessing according to the present invention; and

FIG. 5 is a block diagram illustrating barometric altimeter calibrationprocessing according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference initially to FIG. 1, a navigation device thatincorporates the present invention is denoted generally by referencenumeral 10. Navigation device 10 has a housing 12, a display 14, and aninput 16, preferably a keypad input. Other known inputs, such as a touchscreen, may be utilized additionally or alternatively. The housing 12 ispreferably sized to be portable, although the invention is not limitedto portable units.

With reference to FIG. 2, navigation device 10 has a processor 18.Connected to processor 18 are a memory 20, the display 14, the input 16and a barometric pressure sensor 22. Additionally, a GPS receiver 24 isconnected to the processor 18. An antenna 26, for receiving GPS signals,is connected to the GPS receiver 24. A power source, such as batteries,or a battery pack (not shown), is utilized to supply power to thevarious electronic components. Additionally, navigation device 10 mayinclude a port, such as serial data port, for connecting the device 10to a remote processor or personal computer for uploading information(such as map information) to the device 10, or for downloadinginformation (such as route information) to a remote processor orpersonal computer. Alternatively, the device 10 may include wirelesscommunication capabilities, such that data is received wirelessly from aremote site. As will be understood and appreciated, the variouselectronic components are housed within the housing 12, such thatdisplay 14 and keypad input 16 are accessible at an exterior of thehousing.

With reference to FIG. 3, a graphical representation representing auser's elevation trajectory (A), a GPS elevation reading (B), and abarometric altimeter reading (C) is illustrated. In particular, anexemplary elevation profile of a user using navigation device 10 isrepresented by line A in FIG. 3. In other words, line A represents thetrajectory of a user using navigation device 10 as, for example, he orshe travels over terrain. The GPS elevation reading is depicted bysignal B. As illustrated, the GPS elevation reading B varies over time,as the user moves along the path of use. Similarly, the barometricaltimeter reading varies over time from the actual elevation trajectoryof the user, although far less significantly, typically, than the GPSelevation reading. Additionally, as illustrated, the barometricaltimeter reading is typically offset by a bias amount, or difference D,from the GPS elevation reading.

With reference now to FIGS. 4 and 5, a method of calibrating thealtimeter of device 10 is illustrated and described. In accordance withan aspect of the invention, when the processor determines that thedifference between the altitude based upon a barometric pressure readingfrom sensor 30 and GPS derived altitude differs by a selected thresholdamount, the processor begins computations necessary to calibrate thebarometric readings. In practice, a barometric altimeter typicallyprovides a more stable measurement of altitude than GPS over short timeperiods (from tens of minutes to several hours). However, over long timeperiods, pressure variations can be of such magnitude that thebarometric altimeter measurement of altitude is less accurate that theGPS measurement. As such, the processor must determine the appropriatealtitude, utilizing a combination of these measurements. In particular,as indicated at step 30, the device is started up and initialized.Processing advances to step 32, at which the processor 18 measures thedifference between an elevation reading provided from the barometricaltimeter and an elevation reading provided by the GPS unit. Processingadvances to step 34, at which the processor 18 computes the averagebetween that difference. As will be understood and appreciated, upon theinitial measurement at step 32, the average difference will simply beequal to the difference. As will be further understood, and appreciated,GPS based and barometric altimeter based measurements are takencontinuously, or periodically, and on additional passes through thisprocessing loop, the additional information at step 34 is averagedrecursively, although other averaging techniques may be employed.

A processing advances to step 36, processor 18 computes the uncertaintyof the average difference determined at step 34. A decision statistic isemployed to make the decision to use the estimated barometer altimeterdifference to calibrate the baro-altimeter reading.

At step 38, the processor 18 determines an average barometer drift foran elapsed time associated with the computed average differencedetermined at step 34. In particular, memory 20 preferably has, in tableform, the average drift of the barometer over time. Following step 38,the processing advances to step 40. At step 40, processor 18 determineswhether the uncertainty of the computed average difference, σ_(ΔH)_(ave) , (k), as determined at step 36, is significantly less than theuncertainty due to baro drift, σ_(Baro), as obtained at step 38. Inparticular, it is determined whether the uncertainty of the computedaverage difference is less than the determined average barometer driftby a selected threshold. When the uncertainty of the computed averagedifference σ_(ΔH) _(ave) (k) is not less than σ_(Baro) by at least thethreshold amount, processing returns to step 32, so that the processormay continue taking difference measurements. When, however, it isdetermined at step 40 that the uncertainty of the computed averagedifference is less than a preselected threshold than the obtainedaverage barometer drift, processing advances to step 42.

At step 42, processor 18 determines whether the elapsed time, that beingthe time associated with the measurements taken thus far, is greaterthan a scaled correlation time of GPS vertical errors. When it isdetermined at step 42 that the elapsed time is not greater than a scaledcorrelation time of GPS vertical errors, processing returns to step 32.When, however, it is determined that the elapsed time is greater than ascaled correlation time of GPS vertical errors, processing advances tostep 44. At step 44, the processor 18 calibrates the barometricaltimeter, pursuant to the processing flow of FIG. 5 as discussed below.Following the calibration at step 44, processing advances to step 46,where the error statistics of the average difference are reinitiatedbased upon calibrated barometric altimeter and processing then returnsto step 32.

With reference particularly to FIG. 5, calibration of the barometricaltimeter is illustrated and described. At step 48 of FIG. 5, the device10 of the present invention computes a preliminary calibratedbarometrically determined elevation.

The calibration process begins with obtaining a calibrated barometricaltitude H_(B,cal) by subtracting the estimated calibration altitudedifference from the current barometric altitude i.e.H_(B,cal)(t)=H_(B)(t)−ΔH_(cal) to remove the bias and thus approximatethe true altitude H_(T)(t). This is the approach taken in prior artwhich is not an optimal approach. The present invention goes furtherthan prior art, by using the calibrated barometric altitude H_(B,cal) tocompute a base calibration pressure P_(B,cal), which is then used tocompute local altitude H_(B).

Processing then advances to step 50, where the processor 18 computes acalibrated base pressure value P_(B). The calibrated base pressure P_(B)is determined by solving the following equation for P_(B), identified asP_(B,cal).$P_{B,{c\quad{al}}} = \frac{P_{L}}{\left\lbrack {\frac{H_{B,{c\quad{al}}}*L}{T_{O}} + 1} \right\rbrack^{\frac{- g}{RL}}}$

At step 52, processor 18 computes a calibrated barometric elevation foruse in subsequent measurements. In particular, a calibrated barometricelevation, utilizing the computed calibrated base pressure value,P_(B,cal), is determined according to the following equation:$H_{B,{c\quad{al}}} = {\frac{T_{O}}{L}\left\lbrack {\left( \frac{P_{L}}{P_{B,{c\quad{al}}}} \right)^{\frac{- {RL}}{g}} - 1} \right\rbrack}$

The calibrated barometric elevation is then displayed on display 14 ofdevice 10, and used in further processing.

Accordingly, the present invention employs a method of estimating thebarometric bias using the difference term ΔH_(ave) to common mode outany dynamical changes due to movement of the baro-altimeter and the GPSin tandem which is unique with respect to known methods. In other words,the method of the present invention accounts for the fact that changesin altitude by a user are reflected in both the barometric altitudereading and the GPS altitude reading, thereby allowing calibration totake place while the baro-altimeter and GPS are in motion. A user is notconstrained to be motionless during the “calibration mode”. Furthermore,this method allows the barometric error to be continuously estimated andused to calibrate the system when such a need is determined by thepreviously discussed calibration decision model.

The best known mode for carrying out the present invention utilizesmodels as described below.

Barometric Model

The barometric altitude is modeled byH _(B)(t)=H _(T)(t)+B _(B)(t)+Q _(B)(t)  (1)where H_(B)(t) is the barometric pressure indicated altitude, H_(T)(t)is the true altitude in MSL, B_(B)(t) is a slowly varying bias-liketerm, and Q_(B)(t) is a zero mean Gaussian noise term of variance σ_(Q)². Equation (1) shows that indicated barometric altitude is the sum ofthe true altitude, plus a bias-like term that is due to the pressurevariation of local pressure from the standard atmospheric model, and anoise term that is due to noise of the sensor, A/D, quantization, andother sources.

In order to calibrate the barometric altimeter, the bias term B_(B)(t)must be determined.

GPS Model

-   -   GPS altitude is modeled by        H _(G)(t)=H _(T)(t)+B _(G)(t)+C _(G)(t)  (2)        where H_(G)(t) is the GPS altitude (in MSL), B_(G)(t) is a        slowly varying bias term due to ionospheric errors, ephemeris        errors, satellite clock errors, and other factors, and C_(G)(t)        is a zero mean correlated noise term of a much shorter time        constant than either B_(G)(t) or B_(B)(t). The variance of the        C_(G)(t) process is σ_(V,GPS) ² and is an estimate of the errors        associated with the vertical channel. When Selective        Availability was in operation, C_(G)(t) was the largest        contributor to GPS altitude error (by far). Also, the B_(G)(t)        term is typically much smaller magnitude than the B_(B)(t) term.

Determining Error in Barometric Altitude

One approach to calibrating the baro-altimeter using GPS is to simplyperform a difference of equations (1) and (2) at a particular point intime where certain statistical rules (to be discussed later) are met.H _(B)(t)−H _(G)(t)=ΔH(t)=B _(B)(t)+Q _(B)(t)−B _(G)(t)−C _(G)(t)  (3)Taking the expected value of ΔH(t) yieldsΔH(t)=B _(B)(t)−B _(G)(t)  (4)since the expected value of terms Q_(B)(t) and C_(G)(t) is zero (theyare zero mean noise processes).

One can then calibrate the baro-altimeter using ΔH(t). Calibration isdiscussed in more detail later.

Because Q_(B)(t) and C_(G)(t) are zero mean random processes, one canreduce the error involved estimating B_(B)(t) by averaging ΔH(t) overmany samples. Note that B_(G)(t) is ignored since it is typically small.When estimating a random bias in the presence of additive noise, thevariance of the estimate is reduced by the number of samples used toform the estimate only if the additive noise is uncorrelated. Q_(B)(t)is indeed uncorrelated Gaussian noise. However C_(G)(t) is correlatednoise with a correlation time of τ_(C). Therefore the estimation erroris treated differently.

One can recursively average ΔH(t) over many samples according to eqn.(5).ΔH _(ave)(k)=k−1/kΔH_(ave)(k−1)+1/kΔH(t _(n))  (5)

The uncertainty of this average estimate is the root sum square ofuncertainty reduction in σ_(Q) and the uncertainty reduction toσ_(V,GPS) eqn. (6). $\begin{matrix}{{\sigma_{\Delta\quad H_{ave}}(k)} = \left( {\frac{\sigma_{Q}^{2}}{k} + \frac{\sigma_{V,{GPS}}^{2}}{1 + \frac{k*\Delta\quad t}{\tau_{C}}}} \right)^{1/2}} & (6)\end{matrix}$

where k is the number of samples in the average, and Δt is the intervalbetween samples.

Note that the contribution of σ_(V,GPS) is reduced according to thecorrelation time of this process. What this means is that essentiallyone correlation time must elapse before samples of the C_(G)(t) processare sufficiently decorrelated to contribute an uncertainty reductionequivalent to an independent sample.

It is noted that σ_(V,GPS) is a dynamically changing function, whereasσ_(Q) is a quantity that is chosen a-priori. To accommodate thesedynamics, σ_(V,GPS) is also recursively computed over the estimationinterval. Again, the same weighting function as used in eqn. (5) is usedhereσ_(V,ave)(k)=k−1/kσ _(V,ave)(k−1)+1/kσ _(V,GPS)(t _(n))  (7)

Then σ_(V,ave)(k) is substituted into equation (6). $\begin{matrix}{{\sigma_{\Delta\quad H_{ave}}(k)} = \left( {\frac{\sigma_{Q}^{2}}{k} + \frac{\sigma_{V,{ave}}^{2}(k)}{1 + \frac{k*\Delta\quad t}{\tau_{C}}}} \right)^{1/2}} & (8)\end{matrix}$

Equation (8) is the final form of the uncertainty estimate for ΔH_(ave).

Decision Statistics for Calibrating Baro-Altimeter

The primary decision statistic to use the baro-altimeter differenceestimate ΔH_(ave)(k) to calibrate the baro-altimeter is whenσ_(ΔH) _(ave) (k)<β*σ_(Baro)(t _(n)-t_(cal))  (9)where β is any non-negative constant and σ_(Baro)(t_(n)−t_(cal)) is anestimate of the uncertainty to the baro-altimeter. σ_(Baro) is afunction of the time that has elapsed since the last calibration and theuncertainty of the calibration. Furthermore, k is constrained so thatk*Δt>α*τ _(C)  (10)which constrains the averaging period to be some multiple α of thecorrelation time of C_(G)(t).

Also, B_(B)(t) does vary slowly with time. If the averaging periodexceeds another time threshold, t_(max), the assumption that B_(B)(t) isconstant does not hold, and the averaging process is re-initialized.

Once the calibration decision statistics have been met, ΔH_(ave)(k) isset equal to ΔH_(cal), and all recursive estimation algorithms arere-initialized, in particular σ_(Baro) is set equal to σ_(ΔH,ave).

Approach to Calibrating Barometric Altimeter

The simplest approach to calibration of the baro-altimeter is to simplysubtract ΔH_(cal) from the current barometric altitudeH _(B,cal)(t)=H _(B)(t)−ΔH _(cal)(11)

This removes the bias B_(B)(t) and thus H_(B,cal)(t) approximatesH_(T)(t) the true altitude.

Indeed, this is the approach taken in McBurney, et al. However, this isnot the optimal approach. A fundamentally different approach to barocalibration is used in this invention.

Standard Atmosphere Model Relating Pressure to Altitude

For altitude below 11,000 meters, the following equation is used tocompute altitude from pressure. $\begin{matrix}{H_{B} = {\frac{T_{O}}{L}\left\lbrack {\left( \frac{P_{L}}{P_{B}} \right)^{\frac{- {RL}}{g}} - 1} \right\rbrack}} & (12)\end{matrix}$where the following quantities are define in the 1993 ICAO StandardAtmosphere Model.

-   -   T_(O)=Standard Temperature at Sea Level    -   L=Lapse rate    -   R=Gas Constant    -   g=Acceleration of Gravity    -   P_(L)=Local Pressure (measured by barometer)    -   P_(B)=Base pressure (in this case pressure at Sea Level)    -   H_(B)=Local pressure altitude

From eqn. (12), it is shown that the model that relates pressure toaltitude is an exponential model, not a linear model. Thus, if onedetermines that ΔH_(cal) is 30 meters at a nominal altitude of 500meters, it does not hold that the proper calibration factor will stillbe 30 meters at a nominal altitude of 5000 meters. The reason is that 30meters of elevation difference at 500 meters nominal altitude is a fargreater pressure differential than 30 meters of elevation difference at5000 meters nominal altitude. The calibration method employed in theinvention accounts for this discrepancy.

Barometric Altimeter Calibration Technique

In this invention during calibration P_(L) is treated as a constant andP_(B) is allowed to vary. The newly computed P_(B,cal) (see eqn. 13) isused in subsequent altitude computations in equation (12).

P_(B,cal) is computed as shown in equation (13). H_(B,cal) is thecalibrated barometric altitude estimated using GPS. $\begin{matrix}{P_{B,{c\quad{al}}} = \frac{P_{L}}{\left\lbrack {\frac{H_{B,{c\quad{al}}}*L}{T_{O}} + 1} \right\rbrack^{\frac{- g}{RL}}}} & (13)\end{matrix}$

Summary of Barometric Altimeter Calibration Process

1. Evaluate equations (9) and (10) to determine if calibration decisionstatistics are met.

2. If so, compute H_(B,cal) according to eqn. (11).

3. Compute P_(B,cal) according to eqn. (13).

4. Begin computing H_(B) according to (12) using P_(B,cal).

5. Set σ_(Baro) to σ_(ΔH,ave).

6. Resume estimation of ΔH_(ave) and σ_(V,ave)(k) using eqns. (5-7).

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects herein above set forth togetherwith the other advantages which are obvious and which are inherent tothe structure.

It will be understood that certain features and sub-combinations are ofutility and may be employed without reference to other features andsub-combinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative, and not in a limiting sense.

1. An altitude determining apparatus comprising: an altimeter operableto determine an altitude of the apparatus; a GPS unit for receivingsatellite signals from a plurality of satellites and operable todetermine an altitude of the apparatus; and a processor operable tocalibrate the altimeter accounting for changes in local atmosphericpressure when the altimeter and GPS unit are moved in tandem.
 2. Theapparatus as set forth in claim 1, wherein the processor calibrates thealtimeter while the apparatus is in motion.
 3. The apparatus as setforth in claim 1, wherein the processor continuously calibrates thealtimeter.
 4. The apparatus as set forth in claim 1, wherein thealtimeter, the GPS unit, and the processor are housed in a portablehousing.
 5. The apparatus as set forth in claim 1, wherein thealtimeter, the GPS unit, and the processor are housed in a handheldhousing.
 6. The apparatus as set forth in claim 1, wherein thealtimeter, the GPS unit, and the processor are housed in a portable andhandheld housing.
 7. The apparatus as set forth in claim 1, wherein theprocessor determines a difference in the altitude as determined by thealtimeter and the altitude as determined by the GPS unit.
 8. Theapparatus as set forth in claim 7, wherein the processor calibrates thealtimeter by subtracting the difference from the altitude as determinedby the altimeter.
 9. The apparatus as set forth in claim 1, wherein theprocessor determines an average difference in the altitude as determinedby the altimeter and the altitude as determined by the GPS unit over aperiod of time.
 10. The apparatus as set forth in claim 9, wherein theprocessor calibrates the altimeter by subtracting the average differencefrom the altitude as determined by the altimeter.
 11. The apparatus asset forth in claim 1, wherein the altimeter includes a barometricpressure sensor.
 12. An altitude determining apparatus comprising: analtimeter for determining a first altitude reading; a GPS receiver forreceiving satellite signals from a plurality of satellites; and aprocessor coupled with the altimeter and the GPS receiver forcalculating a second altitude reading as a function of the satellitesignals, such that the processor is operable to calibrate the altimeterwhile the apparatus is in motion by accounting for dynamic changesbetween the readings.
 13. The apparatus as set forth in claim 12,wherein the processor continuously calibrates the altimeter.
 14. Theapparatus as set forth in claim 12, wherein the altimeter includes abarometric pressure sensor.
 15. The apparatus as set forth in claim 12,wherein the processor determines a difference in the altitude readingsand subtracts the difference from the first reading.
 16. The apparatusas set forth in claim 12, wherein the processor determines an averagedifference in the readings and subtracts the average difference from thefirst reading.
 17. The apparatus as set forth in claim 12, wherein thealtimeter, the GPS receiver, and the processor are housed in a portableand handheld housing.
 18. An altitude determining apparatus comprising:a barometric pressure sensor for determining a first altitude reading; aGPS receiver for receiving satellite signals from a plurality ofsatellites; and a processor coupled with the barometric pressure sensorand the GPS receiver for calculating a second altitude reading as afunction of the satellite signals, such that the processor is operableto calibrate the barometric pressure sensor accounting for changes inlocal atmospheric pressure while the apparatus is in motion usingdifferences in the readings.
 19. The apparatus as set forth in claim 18,wherein the processor continuously calibrates the barometric pressuresensor.
 20. The apparatus as set forth in claim 19, wherein thebarometric pressure sensor, the GPS receiver, and the processor arehoused in a portable and handheld housing.